Fig. 1.

Flowchart showing the selection process of relevant studies. In addition to 707 PubMed results identified through database searching, 42 records including quotations, 5 editions of the WHO classification of brain tumors, and recent studies known to the authors were screened. In 182 excluded cases, the abstracts did not provide detailed information about methods and results and full texts were lacking (*). Moreover, 24 full texts could not be considered, since they were not written in English (**).